1. Field of the Invention
The present invention relates to a semiconductive roller suitably employable as a developing roller or a charging roller in an image forming apparatus, such as a laser printer, an electrostatic copier, a plain paper facsimile or a composite machine thereof, for example, utilizing electrophotography.
2. Description of Related Art
Various types of image forming apparatuses utilizing electrophotography are increasingly improved, in order to satisfy requirements for speed increase, improvement in picture quality, colorization and downsizing.
The key to such improvements is toner. In other words, refinement of the toner, uniformization of the particle diameter of the toner, and sphericalization of the toner shape are necessary, in order to satisfy the requirements.
As to the refinement of the toner, fine toner having an average particle diameter of not more than 10 μm or not more than 5 μm has been developed. As to the sphericalization of the toner shape, toner having sphericity exceeding 99% has been developed.
In order to further improve the quality of formed images, polymerized toner is increasingly employed in place of the conventional pulverized toner. The polymerized toner exhibits extremely excellent dot reproducibility particularly in imaging of digital information, to enable formation of high-quality images.
In an image forming apparatus, a charging roller for uniformly charging a surface of a photosensitive body or a developing roller for developing an electrostatic latent image formed by exposing the charged surface of the photosensitive body into a toner image is employed.
A roller including a roller body made of a crosslinked substance of a rubber composition prepared by blending an electronic conductivity supplier such as conductive carbon black into a rubber component and a shaft made of a metal or the like inserted into the center of the roller body, for example, is generally employed as the developing roller or the charging roller.
In particular, a semiconductive roller having roller resistance adjusted to not more than 108Ω is effectively employed as the developing roller, in order to supply high chargeability to the toner in response to the refinement of the toner, the uniformization of the particle diameter of the toner and the sphericalization of the toner shape or the transition to the polymerized toner, and in order to efficiently develop the electrostatic latent image into the toner image without adhering the toner to the roller body.
Further, the semiconductive roller having the roller resistance adjusted in the above range is effectively employed also as the charging roller, in order to effectively charge the surface of the photosensitive body or the like with the minimum power consumption in a short time.
In order to satisfy various requirements of the semiconductive roller, studies are conducted as to the type of the rubber component constituting the rubber composition as well as the type, the compounding ratio and the structure of an additive, for example.
In order to manufacture the semiconductive roller with the highest possible productivity at a low cost, for example, the roller body is preferably nonporously formed in a single-layer structure.
In order to form a high-quality image by suppressing reduction in the quantity of charge of the toner in a case of employing a semiconductive roller including such a nonporous roller body having a single-layer structure as a developing roller, employment of ion-conductive rubber such as chloroprene rubber or epichlorohydrin rubber, for example, as a rubber component is studied.
If a semiconductive roller including a roller body made of a rubber composition containing ion-conductive rubber as a rubber component is used as a developing roller in practice for forming an image, however, the density of the formed image is reduced due to adhesion of the toner to the roller body.